Fun gift for 3.5 yr old

At that age my DSs were obsessed with dressing up costumes. DS1 was a policeman for a few years and DS2 was a pirate, as well as spiderman, doctor etc. You could find out who his favourite characters are and getting him a costume. Nothing beat dressing up in those days.

Or a mini scooter, a wigwam/play tent. These are the things that really get played with. Or Duplo (big Lego for small children.) They absolutely loved Duplo and still played with it until they were about six or seven when they graduated onto Lego.